Anthropology 130 (Online)
Human Variation and Adaptation
By Bonnie Yoshida
What does it mean to take an evolutionary perspective on human
variation? It means that anthropologists are interested in viewing
human traits through the lens of adaptation. Adaptation is the
successful interaction of a population with the environment.
Humans have different levels of adaptation to the environment
depending upon the circumstances. This diagram is a good way of
depicting the different layers of insulation from the environment.
You can think of the outside environment as conditions that cause
stress (red arrows). These can be abiotic or non-living forces such
as climate or altitude. There are also biotic or biological
stressors. These can be disease or a crop failure which results in
food shortage. Or even social stresses like a state of war or the
psychological stresses of living in modern society. All these things
put stress on the individual, causing discomfort, illness or death.
Hierarchy of human responses to stress
alone depend on culture and technology to respond to environmental
stress. Cultural responses can be instituted immediately and are
learned behavior (If youíre cold you can put on clothes, build fire,
go inside, have some soup). Remember the difference between modern
humans and Neandertals in their response to the harsh ice age
environment? Neandertals adapted more with their bodies, but
modern humans used culture and technology such as bone needles to
Immediate physiological responses (Acclimation)
Immediate physiological responses (shivering, sweating, etc). These
are things your body does immediately to respond to a stressor.
Goose bumps for example is a way for your body to conserve heat by
constricting the blood vessels. Once the stress is eased, these
processes are quickly reversible.
These are still short term physiological responses, that are
reversible, but they donít take effect immediately (perhaps taking
days to months to institute and reverse). These processes are
known as acclimatization. Tanning and bodybuilding are some examples
If you stress your muscles to the limit by lifting weights, you will
be able to increase your weight load and build muscle mass.
However, this increased muscle mass will not last--if the weight
training ceases, your body will return to its original state.
Developmental responses occur during the period of childhood growth.
Sometimes this is known as
or phenotypic plasticity. The capacity of humans to respond to
environmental change is
How not only genetics but also our environment determines our
phenotypic characteristics. A poor diet, or poor health care or a
polluted environment can affect your biology. For example, take the
big jump in height during the past hundred years or so. If any of
you are the children of immigrants from non-industrialized
countries, you are probably taller than your parents. This is
probably due to better nutrition and health care in industrialized
Which country has the tallest people in the world today? Read the
The Height Gap (from the New Yorker) to find out.
at the genetic level are changes that happen very slowly over many
generations through natural selection. These can also include
changes in a single allele. The sickle
cell allele is a classic case of a genetic adaptation related to two
different disease processes.
You can read about the Sickle Cell case study on pp. 79-83). It is the
focus of the virtual lab associated with Chap. 4.
Also, check out this web tutorial on the
evolution of sickle cell anemia at the PBS web site.
Genetic responses can also include more complex variation
such as differences in body form, physiology or skin color that
involve multiple alleles and probably a significant degree of
environmental conditioning as well.
A Case Study: Adaptation to High Elevation
High altitude adaptation is a good example of the hierarchy of
responses used by humans to adapt to their environments. There are
anthropologists specializing in human biology who study how some
humans have adapted to living at extreme environments.
|Tenzing Norgay, a Sherpa
||Quechua woman of the Andes region
For example, they may ask why some human populations such as the
Sherpas and mountain people of the Andes above display greater
physiological adaptation to high elevations. The
Sherpas of Nepal are an example of people who have adapted to
living at extremely high altitudes where each lungful of air has
only about a third as many oxygen molecules as at sea level. But how
did they come to attain these capabilities? Do they carry genes that
have evolved to help them survive better at high altitude
conditions? Or does growing up in the environment program the body
to adapt differently than a lowlander would?
When people who live at low elevation travel to high altitudes, they
usually experience immediate stress on the body. Most symptoms
of high altitude stress are caused by hypoxia, which is lack of
Read here to learn more about oxygen transport in the body. This
results in symptoms like headache, nausea, insommia, loss of
appetite, etc. These would be immediate physiological
responses to stress as described above. Have any of you
ever experienced this upon ascent to a high elevation environment?
Eventually, after several days at altitude, you may
acclimate somewhat. This is most likely due to an increase
in your red blood cell count and changes in respiratory patterns and
cell function. However, it is unlikely that you will attain the
equivalent physiological function of someone who grew up at high
elevation. If youíre born in the highlands, you have a somewhat
deeper level of acclimatization at the developmental level.
You tend to have greater lung capacity and be more efficient at
transporting oxygen from blood to body tissues. So they donít need
to form as many red blood cells. Developmental plasticity can also
result in a body form better adapted to high altitude such as
increased lung volume.
What about groups like the Sherpas or highland people in the South
American Andes who have lived at extreme elevations for thousands of
years? Have they evolved still deeper adaptations at the genetic
level to high altitude?
Check out this
National Geographic web article about current research on
human adaptation to high elevation.
Variation in Body Form
Now we will examine two areas where human phenotypic variation seems
quite pronounced--variation in body form and in skin color. As
you'll see, both of these can be explained to some extent by
adaptation to particular environmental circumstances.
Anthropologists have noticed that there seems to be a relationship
between climate and body size in humans as well as other animals.
Theyíve looked at human body size and shape applying two rules about
animal body form that were developed by biologists--Bergmannís rule
and Allenís rule.
Populations in cold climates tend to have larger body size than
populations in warm climates. In other words, species who occupy a
wide range of environments tend to have larger bodies in cold areas.
Polar bears are larger in body size than brown bears who live in
warmer climates. Why? Because a larger body loses heat less rapidly
than a smaller body size. Why is that? Because a larger object has a
smaller surface area to volume ratio. So as body size increases,
volume increases at a faster rate than surface area (as you can see
with the cubes below).
Illustration of Bergmann's rule
Allenís Rule refers specifically to the limbs or appendages of
Mammals in cold climates tend to have short, bulky limbs allowing
for less loss of body heat. Mammals in hot climates have long,
slender limbs allowing heat to dissipate easier. This is the same
geometric principleólonger appendages increase surface area (see
Illustration of Allen's rule
So, do these rules hold true with modern humans? It seems to, to
Populations that inhabit hot climates tend to be very linear in
build, and human populations in cold climates tend to be heavier and
You can see Allenís rule in practice with Eskimos who have a
somewhat stockier body build and shorter limbs, as compared with
East Africans like the Masai who have a long and linear body build.
But this doesnít hold true for all people. There are many exceptions
to this rule. Thereís also a lot of variation within populations,
and a lot of this is dependent on cultural variables such as diet.
And developmental plasticity could also play a role in shaping body
form (see above discussion of human height). So thereís probably a
mixture of cultural, physiological and genetic factors at work here.
Variation in Skin Color
Skin color variation is another example of adaptation and natural
selection in human populations. You can see that there skin color
varies with distance from the equator. What is responsible for skin
color is the production of melanin, so variation in skin color is
due to differences in melanin production. The amount of pigmentation
in skin is greatest in the tropics, and lighter in northern areas.
So what explains this variation in skin color? There are several
One with which you are probably familiar with is that dark skin
helps protect against UV radiation that can lead to sunburn and skin
cancer (above). However, one problem with this hypothesis is that
most skin cancers occur relatively late in life, after a personís
reproductive years are finished. In that case, the effect of natural
selection would be limited.
According to another hypothesis, dark skin evolved to protect
against another harmful effect of UV radiationóit destroys stores of
folate (a member of the B-vitamin complex) in the body. Folate is
necessary for normal cell division. Folate deficiency can have
direct effects on fertility: namely, lowered sperm production in men
and neural tube defects in the developing embryo during pregnancy.
If dark skin has such benefits, why wouldnít everyone simply have
dark skin? What selective pressures encouraged the evolution of
light skin? One positive effect of UV radiation is that it
synthesizes vitamin D for us, which is an essential vitamin. We can
get vitamin D from foods, but the main source is from the sun.
Vitamin D is key for mineralizing bones and normal growth. A lack of
vitamin D can lead to rickets which produces soft bones and pelvic
deformities. As people moved from the tropics to more temperate
areas, reduced sunlight and the need to wear more clothes would
select for lighter skin to regulate Vitamin D synthesis.
Children with bowed legs resulting from rickets; X-ray of the
Here are two articles on the evolution of human skin color: